Retatrutide vs Mounjaro Dosage: How Do They Compare?

What receptors do retatrutide and tirzepatide actually act on — and why does it change the dose?

Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. It mimics two gut hormones simultaneously, slowing gastric emptying and reducing appetite through complementary pathways. It is the only dual-agonist weight-loss medicine licensed in the UK, available in six strengths: 2.5, 5, 7.5, 10, 12.5 and 15 mg, each delivered once weekly via a pre-filled KwikPen. Treatment starts at 2.5 mg to help the body adjust (the first pen's job is settling your system, not producing immediate weight change) then a prescriber titrates the dose upward roughly every four weeks based on tolerability and response. If you are ever unsure how your prescribed strength translates to the markings on the pen, our guide to Mounjaro dosage in ml walks through exactly what each dose looks like in practice. The NHS tirzepatide medicines page covers the full prescribing context.

Retatrutide adds a third receptor to that mix: it also agonises GIP and GLP-1 but includes glucagon receptor activity, making it a triple agonist. In phase 2 trials, retatrutide was studied at doses ranging from 1 mg up to 12 mg weekly, with the highest doses producing striking average weight reductions. Those results generated considerable media interest. The catch is straightforward: retatrutide has not completed phase 3 trials, has received no MHRA marketing authorisation, and cannot be legally prescribed or dispensed in the UK for weight loss. Any seller offering it now is operating outside the licensed supply chain.

So when comparing dosages, you are comparing a fully characterised, licensed schedule (Mounjaro's 2.5–15 mg pathway) against phase 2 research numbers that have not yet been validated at scale. They are not equivalent comparisons.

How do the weight-loss results at each dose level actually stack up?

For tirzepatide, the SURMOUNT-1 trial (published in the New England Journal of Medicine and involving 2,539 adults with obesity) reported an average body-weight reduction of around 20–21% at the 15 mg maintenance dose over 72 weeks. That trial underpins NICE's recommendation of tirzepatide (TA1026, December 2024) for eligible adults in the NHS. The 10 mg and 12.5 mg doses also produced clinically meaningful reductions in the trial, which matters because not every patient reaches or tolerates the top dose.

Retatrutide's phase 2 data reported average weight loss of around 17–24% at the higher doses over 48 weeks in a smaller cohort. The absolute numbers look competitive (in some analyses, higher) but phase 2 trials are deliberately smaller and shorter than pivotal phase 3 studies. They are designed to find a signal and narrow the dose range, not to establish the full safety and efficacy picture that regulators need before granting a licence. Comparing a NICE-appraised phase 3 result with a phase 2 signal is a bit like comparing a finished building to an architect's drawings. The drawings may look impressive; the building is the one you can move into.

Our prescribers are asked most weeks whether patients should wait for retatrutide instead of starting Mounjaro. The honest answer is that nobody knows when, or whether, retatrutide will reach UK patients — and in the meantime, people are missing months of a treatment that already works.

Is anything about the retatrutide vs Mounjaro comparison relevant to a UK patient today?

One thing is genuinely relevant right now: understanding that Mounjaro is the licensed triple-pathway-like option available in the UK. Because it activates both GIP and GLP-1 receptors, it already goes further than a single-pathway GLP-1 medicine. The retatrutide and tirzepatide dosage chart sets out the phase 2 dose comparisons in detail if you want the numbers side by side.

For patients already on Mounjaro wondering whether switching will ever make sense, the guidance is clear: that decision belongs with a prescriber, and it will depend on data that does not yet exist at the level regulators require. The question of switching from Mounjaro to retatrutide covers what that transition might look like if and when a licence arrives. For now, the practical path runs through Mounjaro's existing six-dose schedule. If you want to understand where you would sit on that schedule, the full Mounjaro dosage guide explains the titration logic your prescriber follows.

Mounjaro is a prescription-only medicine. Under-18s, people who are pregnant, breastfeeding or trying to conceive should not use it. A history of medullary thyroid carcinoma or MEN2, or certain gastrointestinal conditions, requires careful prescriber discussion before treatment begins. The NICE TA1026 recommendation sets out the NHS eligibility criteria in full.

How does the dosage comparison translate into what you can actually access?

The table makes the position plain. Retatrutide's dosing data is real and the results are scientifically interesting, but interesting is not the same as available. Mounjaro's schedule is established, its risks are characterised, and a prescriber can walk alongside you through every dose step. Which medicine suits your health picture is a clinical decision our prescribers make with you. To start that conversation, you can begin your free consultation today. If cost is part of your thinking, the Mounjaro cost guide explains what a legitimate private prescription includes. And if side effects at the higher steps concern you, the 12.5 mg side effects page covers what the evidence says about that specific dose.